[Mesa-dev] glBufferSubData() GPU stall reduction (DOTA2 optimization).

[Eric Anholt]

Since it sounds like valve won't be able to fix dota2's rendering to use
ARB_mbr soon, here's a series to add just a little bit of tracking that
works around most of the overhead of not using ARB_mbr with their
rendering pattern.  7.69854% +/- 0.909163% (n=3) fps improvement with
default settings.  We could also leverage this for some apps that misuse
ARB_mbr in the future.

This doesn't look like it affects GLB2.7, which has a very special-looking
access pattern to its BO.

Code is also on the "subdata" branch of my tree.